Because they’re not “green” enough:

Big, boxy Zambonis are the most popular ice-grooming machines, though they are not the only brand on the market. In 1967, in Elmira, Ontario, a welder named Andrew Schlupp built his own ice-resurfacer and started the Resurfice Corporation.
Now Resurfice has answered the call for a greener, cleaner machine, developing an electric-powered model that will be used at the 2010 Winter Olympics in Vancouver, British Columbia. Don Schlupp, the director of sales and marketing (and Andrew’s son), said Resurfice, which supplies propane-powered resurfacers to five National Hockey League rinks, is hoping to put the electric machines on the market soon.
“It’s one of the pillars of the Olympic movement to be sustainable as much as possible,” Denis Hainault, director of ice sports for the Vancouver Organizing Committee, said.